1. Field of the Invention
The invention relates to a method of testing weld seams using an eddy-current technique, which method can be used, in particular, for fault location in components whose weld metal has a coarse-grained microstructure which cannot be made finer by a heat treatment.
2. Discussion of Background
Very high requirements are imposed on weld seams in relation to freedom from faults. Possible faults are fusion defects (side-wall defects, root defects or interrun fusion defects), pores, cracks (in the weld or alongside it), slag enclosures and undercuts.
The welding is therefore usually followed by a preliminary test before the workpieces are given a stress-relief anneal and the weld seam is subjected to a final test.
Various methods are known for assessing welded joints, for example magnetic stray-flux testing, radiographic irradiation testing and the pulse-echo technique using ultrasound.
The location of internal faults, for example cracks, with the aid of X-ray or gamma radiation presents difficulties in weld seams if the thickness of the weld seam is very great, since only those faults can be detected whose size is at least 2 to 3% of the workpiece thickness. The radiographic method is also ruled out if the weld seam is difficult to reach. In these cases, ultrasonic testing and magnetic stray-flux testing are left as test methods, magnetic stray-flux testing being restricted, however, to magnetizable materials and to the surface region of the test pieces. Consequently, often only ultrasonic testing is open to discussion.
One problem in the ultrasonic testing of workpieces and, consequently, also of weld seams may lie in the grain size. If the wavelength of the ultrasound is very much greater than the grain size, the microstructure grain is, to a certain extent, missed by the sound. If, however, the grain size is approximately 1/10 of the wavelength of the ultrasound or even greater, the scattering of the sound which occurs may render testing of the weld seam impossible. That is the case, for example, for austenitic weld seams.
Specifically, the scattering phenomena not only reduce the level of the echoes, but also generate many smaller echoes which result in an irregular interfering background against which the echoes of the defective points stand out only indistinctly or not at all.
An increase in the ultrasonic power (higher transmission voltage) or a signal amplification provide no improvement in this connection since the interfering background increases to the same extent as the echoes which are of interest. If the test frequency is reduced as an alternative, that is to say the wavelength is increased, then, although microstructural conditions having a somewhat greater grain size can be tested using ultrasound, the detection sensitivity for small defect points decreases.
From practical experience it is known that thick-walled TIG-welded pipes are examined for the occurrence of hot cracks in the weld seam and in the heat-affected zone using optical methods by magnifying and imaging the weld seam on a monitor. The disadvantages of this method are that only those cracks can be detected which extend to the surface and that the inspection is very time-consuming and is very fatiguing for the test operator.
A further known method for determining microstructure inhomogeneities is magnetic induction testing. If a metallic workpiece is introduced into an alternating magnetic field generated by a coil, eddy currents are induced in the workpiece and the said eddy currents generate, in turn, a magnetic field which is opposed to the field of the coil. If there are cracks, pores or other inhomogeneities in the workpiece, the eddy currents have to flow round these obstructions, with the result that the secondary magnetic field and, consequently, the secondary voltage are affected. In the case of long workpieces, for example pipes, the self-comparison method is used, but in this method neither the crack length nor the crack depth can be determined in the case of continuous longitudinal cracks. If exploration coils are used for crack testing, then, although the crack depth can be determined if certain conditions are observed, the method is suitable only for the subsurface region of workpieces and, consequently, not for thick workpieces and thick weld seams.
The abovementioned methods also have the disadvantage that the weld seams can be tested only after their complete fabrication and, consequently, substantial throughput times occur. On-line testing of large-volume weld seams having close correlation with the fault size is unknown.